When a substance is processed by chemical vapor deposition (CVD), reactive ion etching (RIE) or the like, a topography simulation is often pre-performed to calculate changes in topography of the substance associated with the processing. In the topography simulation, the changes in topography of the substance is calculated, for example, by dividing a calculating area for calculating the topography of the substance into a plurality of cells, expressing the topography of the substance using filled cells which are filled with a predetermined amount of the substance and vacant cells which are not filled with the predetermined amount of the substance, and calculating changes in distribution of the filled cells and the vacant cells (this method is referred to as “cell method”). However, the cell method is generally requested not only to improve calculation accuracy but also to reduce calculation time, so that the improvement of the calculation accuracy based on a reduction in cell size and time step is limited in the cell method. Consequently, when the topography of the substance is expressed using the filled cells and the vacant cells, there is a possibility that the changes in topography of the substance cannot be accurately calculated depending on the topography of the substance. Therefore, simulation results obtained by the topography simulation may be inconsistent with the changes in topography of the substance in actual semiconductor fabrication steps.